When welding together the edges of large steel plates to form ship hulls, American Society of Mechanical Engineers Code regulations require that the marginal portions of these plates be continuously heated to a temperature of between 200 and 300 degrees Farenheit prior to, during, and after the process of welding them together. Such heating prior to and during the welding process minimizes the temperature differential in the plates as the welding process is carried out, which in turn minimizes misalignment, warping, buckling or distortion between the plates being welded. Such heating after the welding process is completed serves to minimize distortion in the plates as they cool, thus reducing the amount of internal stress in and around the weld joint. The overall effect of the heating is to produce a stronger, deeper weld joint between steel plates which may be as much as six inches thick.
In the past, this heating process has been effectuated by strip heaters such as those disclosed in Senior U.S. Pat. No. 2,877,332, Norton U.S. Pat. No. 3,045,098, Van Noy, et al. U.S. Pat. No. 3,047,704, Drugmand et al. U.S. Pat. No. 3,207,887, Volker U.S. Pat. No. 3,272,968, Drugmand U.S. Pat. No. 3,444,357 and Tanaka U.S. Pat. No. 3,749,881. A plurality of such strip heaters are typically mounted in tandem along the marginal areas of the plates to be welded. Each of the heaters is then connected to a source of electric current, and in turn heats the marginal plate section abutting it.
Since the heater elements of many strip heaters operate at temperatures of 1000 degrees Farenheit or more, the steady state temperature condition of the plate sections abutting the heaters can attain values well in excess of the 300 degree Farenheit maximum limit set by the ASME Code, despite the large amount of heat conducted away from the marginal plate sections by the thick body of the plate. To avoid this overheated condition, human operators are employed to continuously monitor the temprature of the marginal plate sections with Tempilstik.RTM. or other temperature indicating means, and to continuously cut off and on the supply of electric power to heaters on the verge of overheating or underheating their respective workpieces.
The prior art solution to the problem of maintaining the temperatures of the marginal portions of thick steel plates to within a given temperature range is both expensive and inaccurate. It necessitates the employment of large amounts of unskilled labor to work twenty four hours a day, seven days a week to maintain the marginal plate sections to within the desired temperature ranges. Further, as the work is boring and repetitive, there is ample opportunity for a laborer to overlook a critical temperature reading and to let a portion of a workpiece attain an overheated or underheated condition which in turn detracts from the quality of the resulting weld joint. Moreover, considerable amounts of electrical energy are wasted whenever the error is on the side of an overheated condition, as the heater elements of each heater typically consume power at the rate of 2000 to 3000 watts.
Although automatic temperature controls for welding pre-heaters are generally known in the prior art, none of these controls is suitable for accurately and conveniently maintaining the temperature of the marginal areas of large steel plates one to six inches in thickness to within the limits required by the ASME Code. For example, while Bates U.S. Pat. No. 2,276,643 and Smith U.S. Pat. No. 2,184,534 both disclose automatic temperature controls in systems satisfactory for preheating and annealing objects such as plumbing pipes, the thermocouple sensing means utilized in each of these temperature controls is entirely unsatisfactory for use in a ship plate heater for at least three reasons. First, the thermocouple heat sensors used in these systems must be laboriously affixed to the workpiece itself, which is time consuming and labor expensive, and impedes the portability of the heater units. Second, the relatively delicate thermocouples and their connector lines can be easily damaged in the rough physical environment characteristic of shipyards. Thirdly, because of the broad temperature gradient between the 1000 degree Farenheit heater element and the marginal and adjacent portions of the one to six inch thick steel plate being heated, the thermocouples must be placed in exactly the right position on the marginal plate section or an erroneous temperature reading could result, which in turn would result in inaccurate functioning of the control.
Clearly the prior art has failed to provide a satisfactory temperature control for strip heaters used to heat thick steel plates to within a given temperature attendant to a welding process. Ideally, such a temperature control should be simply and conveniently attachable to prior art strip heaters. Moreover, such a control should not impede the portability of the strip by utilizing heat sensing elements which must be specially imbedded into the workpiece before use. Further, the control should be both sturdy and weatherproof and able to operate under rough shipyard conditions. Finally, such a control should be capable of consistently and accurately maintaining the temperature of the marginal plate section to within a given range, and should be capable of being inexpensively mass produced.